The present invention relates to aircraft noise and, more particularly, to the reducing of such noise.
The procedure to certify that an airframe and engine configuration of a modern aircraft meets the requirements imposed by the International Civil Aviation Organization (ICAO) and the Federal Aviation Administration (FAA) explicitly require that the acoustic emissions therefrom, or noise, either attain or be less than the minimum noise specifications that are part of these requirements. Those specifications are dependent on, among other factors, the gross weight of the aircraft. In recent years, certification that these minimum noise requirements for an aircraft have been met has assumed an increased significance due to the more stringent environmental constraints that have been imposed by the ICAO. Specifically, the introduction of new Stage 4 noise restrictions has motivated airframe and engine manufacturers alike to seek out methods to not only attenuate the generated noise but to also minimize the noise generated at the sources thereof through improved designs.
Various studies have clearly demonstrated that the dominant contributor of noise from an aircraft is the gas turbine engine propulsion system in those aircraft in which they are installed. The noise that emanates from such a system can be classified into tonal and broadband spectral components. Tonal noise, characterized by relatively substantial and isolated peaks in the noise frequency spectrum, is generated by aerodynamic interactions between turbomachine airfoils circumferentially mounted on outer surfaces of rings to thereby form blade rows. The frequency of these spectral peaks is determined by the number of airfoils on a ring, the number of such rings and the rotational speed of the shaft on which these rings are rotatably mounted. From the perspective of the effects of such noise on communities, tonal noise is known to be the more annoying of these noise components, a fact that is accounted for in the methodology used to quantify overall aircraft noise as described below.
The noise emission characteristics of an aircraft for certification purposes are determined using procedures specified by the ICAO. In brief, the frequency spectra of noise generated by the aircraft during a specified trajectory, recorded either during an actual flight test or simulated numerically from static engine noise data projected along the flight path, are used to generate a flight time history of the perceived noise level corresponding thereto. Noise spectra under ICAO procedures are generally measured by a spectrum analyzer at each successive sampling time (e.g. sampling of the sound pressure level every ½ second) over this flight time past a location at a uniformly spaced set of acoustic frequencies (e.g. at selected frequencies separated from adjacent ones by 16 Hz) in a range of acoustic frequencies. An example of such a measured spectrum at one sampling instant is shown in the graph of FIG. 1 as plot 1, the measured spectrum, in which the sound pressure level (SPL) measured in decibels (dB) is plotted against acoustic frequency measured in Hertz (Hz). Such ‘narrow-band’ spectra are converted to equivalent ⅓ octave-band spectra by lumping acoustic energy within 24 logarithmically-spaced acoustic frequency bands in the acoustic frequency range between 50 Hz and 10000 Hz, as defined by the ICAO. A ⅓ octave-band spectrum plot, 2, corresponding to measured spectrum 1 is also shown in the FIG. 1 graph for the same sampling instant.
Such spectra are converted to perceived noise level (PNL) by a weighting procedure that accounts for the annoyance that has been determined to be experienced by the human ear, using corresponding ‘noy’ values which are a function of both the acoustic frequency, measured in Hertz, and the sound pressure level, measured in decibels, again as defined by the ICAO in corresponding tables. A typical example of the variation of noy values over acoustic frequency is shown in FIG. 2 at three different typical sound pressure level values, 3′, 3″ and 3′″, encountered in the estimation of noise generated by aircraft. This weighting procedure involves, in the present example, taking the sound pressure level for each frequency band in plot 2 of FIG. 1 and obtaining, from the ICAO noy tables using interpolation as needed, the corresponding noy value for that frequency band to provide a noy value spectrum. These noy values are then summed across the acoustic spectrum frequency range in the noy spectrum to determine the perceived noise level given in decibels for this sampling instant. This perceived noise level is then required to be modified in value based on the spectral severity of the most significant tone, in accord with a corresponding ICAO procedure, to result in a perceived noise level, tone-corrected, (PNLT) that is also given in decibels. In this procedure, prominent peak noy values are identified through a prescribed complex frequency analysis methodology, as well as the acoustic frequencies at which the value or values occur from the noy spectrum. If these peak noy values protrude above the adjacent spectral portions resulting from the random acoustic energy or noise emissions sufficiently, the value of the required modification of the perceived noise level for this peak, i.e. the penalty, is obtained from the corresponding ICAO tables in decibels and combined with the previously determined value for the perceived noise level to provide the tone-corrected perceived noise level value for this sampling instant. The magnitude of the penalty is based on the frequency at which the noy peak occurs and the amount that the peak exceeds the random noise spectral portions in the adjacent ⅓ octave frequency bands. Determining this penalty involves the rate at which the spectral amplitude changes from the ⅓ octave frequency band in which the peak occurs to those bands adjacent to the peak containing band, that is, the slope of the spectrum in the locality of the peak
The tone-corrected perceived noise level for each sampling instant is summed over the flight time history and averaged thereover through dividing this sum by the total elapsed time in that flight time history to yield an effective perceived noise level (EPNL). This result, again given in decibels, is the measure of emitted aircraft noise signature solely relied upon by the ICAO for each relevant acoustic condition including a) the sideline condition in which there is full aircraft engine power propulsion past the acoustic measuring instrument, b) the cutback condition in which engine power is much reduced as occurs as the aircraft reaches a desired altitude following a takeoff, and (c) the approach condition in which engine power is reduced with wing flaps and landing gear down as occurs for the aircraft approaching a landing. This noise evaluation result will be substantially influenced by the ICAO emphasis on annoyance of those within earshot of the aircraft as indicated in FIG. 2 because of the plots therein showing that tonal peaks in the acoustic frequency range of 3000 to 4000 Hz exhibit relatively large noy values as defined by the ICAO. Any occurrence of a dominant tonal peak in this frequency range will be a very significant contributor to the EPNL value for an aircraft. Sound waves are attenuated as they travel through the atmosphere with this attenuation (characterized by a coefficient, α) being weakly dependent on atmospheric temperature and humidity. This atmospheric attenuation does, however, depend significantly on frequency in a manner such that sound waves at higher frequencies are more attenuated than those at lower frequencies as shown in a plot, 4, in the graph of FIG. 3. Plot 4 is of the atmospheric sound pressure attenuation coefficient versus frequency after it has been normalized by the value of that coefficient at the acoustic frequency of 50 Hz. Thus, any method of increasing the frequency location, i.e. a spectral upward shifting, of a tonal peak in the spectral range given above for relatively large noy values will result in a decrease in the EPNL value for an aircraft by virtue of two separate and independent effects: a reduction in the noy value associated with the tone as can be seen in FIG. 2 for such a frequency location shift, as well as the further benefit provided by the increased atmospheric attenuation resulting from such an upward frequency shift as shown in FIG. 3.
However, increasing or shifting upward the frequency of a tonal peak can also result in a small penalty incurred because of the ICAO tone correction or modification requirement with respect to the perceived noise level indicated above. A tonal peak modification at a specific frequency is based, under the requirement, on the rate of change of the SPL value as a function of frequency, or the local slope of the spectrum. Hence, the contribution by a tone depends not only on the SPL value of the peak of that tone but also on its value with respect to the spectral components in the adjacent ⅓ octave frequency band or bands. Consequently, care needs to be exercised in altering a tonal peak over a portion of the frequency spectrum so as not to incur an offsetting penalty from its corresponding required tone modification. Thus, there is desired a method whereby reductions in aircraft EPNL values are achieved by shifting the frequency of a tone or by reducing its required tone modification, or both, corresponding to changes in the engine configuration, that are simulatable to allow predicting the results of such tonal peak alterations and optimizible to aid in the selection of engine aerodynamic design parameters to attain a desired value of EPNL therefor.